1. Technical Field
The present disclosure relates to a resin composition for printed circuit board, a prepreg for printed circuit board, an insulating substrate, a metal-clad laminate, a printed circuit board, magnesium oxide, and a method for manufacturing magnesium oxide.
2. Description of the Related Art
There is a demand for electronic devices to be reduced in size and weight. On the other hand, with an increase in the information processing amount of electronic devices, it is required to enhance performance of the electronic devices. In order to meet these demands, it is required to improve mounting technologies, such as high integration of electronic components such as semiconductor devices to be installed in electronic devices, and densification and multilayering of wirings.
For example, it is required to more densely mount electronic components on a printed circuit board. In the case of a printed circuit board having electronic components densely mounted thereon, a heat amount generated in per unit area is increased. Further, the use of electronic components having a relatively large amount of heat generation, for example, LEDs (light emitting diodes) results in a significant increase in heat generation.
In order to reduce defects due to the increased heat generation, it is necessary to improve the heat dissipation characteristics of the printed circuit board.
In order to improve the heat dissipation characteristics of the printed circuit board, there is proposed a way of blending a resin component for forming a printed-wiring board with magnesium oxide having high thermal conductivity as an inorganic filler. The magnesium oxide has not only high thermal conductivity but also excellent electrical insulation. Thus, blending of the resin component with the magnesium oxide as an inorganic filler allows for improvement in product stability such as increased insulation between wirings as well as increased heat dissipation of the printed circuit board.
Magnesium oxide particles are categorized into light-calcined magnesium oxide (about 600° C. to 900° C.) and dead-calcined magnesium oxide (about 1,100° C. to 1,500° C.) in accordance with the calcination temperature. The former is used for utilizing the excellent chemical activity for the neutralization of acids and halogens. One typical example of such use is an acid acceptor for halogenated rubbers such as chloroprene and chlorosulfonated polyethylene. The latter is used for utilizing the excellent physical properties of magnesium oxide particles, i.e., high-melting point (about 2,800° C.), high electrical insulation at high temperature, translucency over a wide wavelength range, and high thermal conductivity. Specifically, the latter is used in articles which take advantage of the excellent physical properties of magnesium oxide particles, such as a heat-resistant container, a heat-resistant component, a heat insulating material, an integrated circuit (IC) substrate, a lens, a sodium lamp container, a sheath heater, a filler for resin compositions, and a polishing material.
However, magnesium oxide particles are gradually corroded with water or steam and converted to magnesium hydroxide particles (hydration). Thus, the various excellent physical properties are lost, whereby the scope of its use is limited.
The same holds for a printed circuit board containing magnesium oxide. Even if the heat dissipation is high, the product stability is low. Hence, the conversion of magnesium oxide contained in the printed circuit board to magnesium hydroxide results in expansion of the volume of the inorganic filler. As a result, the resin component may be released from a fiber base member which forms the printed circuit board. The occurrence of releasing of the resin component causes the product stability of the printed circuit board to decrease, for example, damages on the strength and insulation of the printed circuit board.
In order to reduce the occurrence of such defects, the use of magnesium oxide having reduced reactivity is studied. It is proposed that such magnesium oxide is manufactured by the following method.
Unexamined Japanese Patent Publication No. 61-85474 proposes a method for calcining magnesium oxide at 1600° C. or more and less than melting temperature (2800° C.). However, the magnesium oxide calcined by the method forms a large lump. Thus, a vigorous grinding process is necessary to form fine particles. Performing the vigorous grinding process on the magnesium oxide causes breaks in single crystals of magnesium oxide particles finally grown and various lattice defects on the surfaces of the crystals. As a result, the water resistance is not satisfied, the contour becomes irregular, and the flowability is low. Thus, it is difficult to highly fill a resin with the magnesium oxide particles.
Another method for manufacturing a low-reactive magnesium oxide includes, for example, an electromelting method. Magnesium oxide obtained by the electromelting method also forms a relatively large lump. Therefore, there remains the same problem as that in the case of the high temperature calcination as described above. Moreover, the electromelting method causes an increase in cost.
Another way is considered to control a particle size of magnesium oxide to be contained in a printed circuit board. For example, PCT International Publication No. WO 2011/007638 (hereinafter referred to as “PTL”) discloses magnesium oxide particles having a controlled particle size. Specifically, PTL discloses magnesium oxide particles having a ratio of (median diameter)/(specific surface area diameter determined from specific surface area) of 3 or less and a D90/D10 of 4 or less. Further, PLT discloses that the magnesium oxide particles are used as high heat dissipative material.